1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to an array substrate for a liquid crystal display (LCD) device and a manufacturing method of the same.
2. Discussion of the Related Art
In general, the LCD device is composed of two substrates, which are spaced apart and facing each other, and liquid crystal interposed between the two substrates. Each of the substrates includes an electrode and the electrodes of each substrate are also facing each other. Voltage is applied to each electrode and an electric field is induced between the electrodes. An arrangement of the liquid crystal molecule is changed by the intensity of the electric field, and the LCD device plays a picture by transmissivity of the light varying according to the arrangement of the liquid crystal molecule.
Because the LCD device is not luminescent, it needs an additional light source in order to display images. Accordingly, the LCD device has a back light behind a liquid crystal panel as a light source. An amount of light incident from the back light is controlled according the alignment of the liquid crystal molecules to display images. The electrodes of each substrate are formed of transparent conductive material and the substrates must be transparent. The LCD device like this is called a transmissive liquid crystal display (LCD) device. Because the transmissive LCD device uses an artificial light source such as the back light, it can display a bright image in dark surroundings. However, the transmissive LCD device has high power consumption.
The reflective liquid crystal display (LCD) device has been suggested to overcome the power consumption problem of the transmissive LCD device. Because the reflective (LCD) device controls transmittance according to the arrangement of liquid crystal molecules depending on applied voltage and by irradiating light using an external light source such as ambient light or artificial light, it has a low power consumption compared with the transmissive (LCD) device. An electrode of the lower substrate is formed of conductive material, which has a high reflectance and an electrode of the upper substrate is formed of transparent conductive material to transmit the incident light.
On the other hand, the reflective LCD device includes a thin film transistor as a switching element. Amorphous silicon is widely used as an active layer of the thin film transistor because it can be uniformly formed at a low temperature over a large area. However, the amorphous silicon is sensitive to visible light. That is, when a light is absorbed into the active layer of the thin film transistor, a leakage current due to the absorbed light flows in the thin film transistor. This leakage current causes an undesirable signal in the LCD device, so that the thin film transistor cannot properly function as a switching element. Therefore, a black matrix, which shields the thin film transistor from the light, is formed on a substrate opposing the substrate having the thin film transistor facing the thin film transistor. However, it is difficult to completely shield the light by the black matrix because accurate arrangement of the black matrix and the thin film transistor is not easy. If light is entirely shielded, the black matrix should have a larger size than the thin film transistor in consideration for alignment margin. Therefore, aperture ratio of the LCD device is reduced.
Various structures of an array substrate for a reflective LCD device are proposed in order to solve the above problem. An example of the array substrate of the conventional reflective LCD device will be described hereinafter in detail with reference to FIG. 1.
FIG. 1 shows a cross section of an array substrate of the conventional reflective LCD device. In FIG. 1, a thin film transistor “T”, which comprises a gate electrode 4, an active layer 8, a source electrode 12, and a drain electrode 14, is formed on a substrate 1. The active layer 8 exposed between the source electrode 12 and the drain electrode 14 is a channel “CH” of the thin film transistor “T”. The substrate 1 is made of an insulating material such as glass. A passivation layer 16 is formed on the thin film transistor “T”. The passivation layer 16 is made of an organic material such as a benzocyclobutene (BCB) and an acrylic resin or an inorganic material such as silicon nitride (SiNx) and silicon oxide (SiO2). The passivation layer 16 has a contact hole 18 which exposes a part of the drain electrode 14. A reflective electrode 20 is formed on the passivation layer 16. The reflective electrode 20 contacts the drain electrode 14 through the contact hole 18. The reflective electrode 20 acts as both an electrode, which drives a liquid crystal molecule, and a reflector, which reflects incident light. Here, the reflective electrode 20 covers the thin film transistor “T”, so that the incident light does not get to the channel “CH” of the thin film transistor “T”. And also the brightness of the LCD device improves because the reflective area becomes wider.
However, when voltage is applied to the reflective electrode 20, the reflective electrode 20 acts like another gate electrode. Therefore, the thin film transistor “T” operates abnormally due to the dual gate phenomenon.
To solve the problem, a structure of an array substrate for a reflective LCD device is suggested in U.S. Pat. No. 5,500,750. FIG. 2 is a cross-sectional view of showing a part of the array substrate for the conventional reflective LCD device illustrated in U.S. Pat. No. 5,500,750. Here, the same referenced symbols used in FIG. 1 is given to the same do part as the conventional array substrate of FIG. 1.
In FIG. 2, a light shield film 22, which is isolated electrically from the reflective electrode 20, is formed right above the thin film transistor “T” in order to shield the channel “CH” from light. Since the light shield film 22 is disconnected from the reflective electrode 20, no electric charges are created in the light shield film 22 and no electric field is induced between the light shield film 22 and the thin film transistor “T”. Therefore, the thin film transistor “T” operates normally.
However, the light shield film 22 and the reflective electrode 20 should have a gap between them in order that the light shield film 22 should be disconnected with the reflective electrode 20. The width of the gap should be over at least 4 μm, which is the minimum value conventionally. Accordingly, the aperture ratio of the conventional reflective LCD device decreases by a size of the gap.
Moreover, the reflective LCD device cannot be used in a dark place because it relies on an external light source.